Emerging research has revealed that, aside from net primary productivity, plant functional traits are reliable predictors of cover crop ES . Advancements in the understanding of the linkages between multi-functionality and plant functional diversity open new opportunities to elaborate cover crop seed selection rationales . This highlights new opportunities to optimize cover crop selection, to meet stakeholders’ desired outcomes. The widespread adoption of cover cropping is delayed by sets of catch-22s. With limited economic compensation for cover cropping, seed costs constitute an important opportunity cost for farmers, leading research to focus on low-cost mixes of one to two species. We observe a noticeable lag in development of new cover crop cultivars, despite the need for improved seed varieties. Poor germination, winter-kill and indeterminate flowering limit the beneficial outcomes of unimproved seed varieties. The use of simplified mixtures in cover cropping restrains our capacity to capitalize on the synergetic effects of higher diversity for ES delivery, predicted by Tilman’s diversity-productivity theory . Thereby, the transition towards cover cropping is held by a valuation-optimization bind, whereby development is constrained by lack of perceived value and value is limited by lack of optimization. Considering the socio-cultural process of catch-22s, dutch buckets system addressing these complexities will require recognizing shared priorities and negotiating a common vision, across differential value systems.
We propose that reaching a consensus over the rationale of cover cropping for sustainable agriculture requires that decision processes in seed selection be elucidated and integrated within the broader sustainability discourse. With clear and comprehensive systems of ES valuation, optimization can integrate experiential feedback to scientific knowledge, and develop seed selection, which reflects practitioners’ system based objectives. This will require the important engagement of the seed industry to establish breeding programs and regional optimization trials, as well as the work of seed distributors to secure widespread availability of improved cover crop varieties.Our keyword selection procedure had limitations. Future research would need to verify the representativeness of our subset of studies. There likely exist additional relevant cover crop research articles for perennial systems. These articles may have used the terms “cover-crops”, “cover crops”, “soil health practice”, “soil management practice”, “conservation practice” to refer to the same practice. Our selection process may not have collected these articles. There likely exist more than 285 studies reporting on this subject. In total, research investigated cover cropping effects across 44 commodities and 36 countries. Although these data provide important insight into the distribution of this research across the agricultural landscape, it may not reflect countries’ respective contribution to the research, as lead and/or partnering institutions, nor does it report on the distribution of research between industry, governmental and university sectors. It is important to note that our database presented a regional data gap, as there were no studies conducted in Russia.
Prior literature indicates that considerable work was conducted by researchers of the Soviet Union on the role of cover crops in supporting biological control in orchards . In particular, we were unable to access works of Telenga and Chumakova . Furthermore, although some of the literature included was in Portuguese, Chinese and French, the vast majority of literature included in this study was in English and therefore may not have encompassed the full breadth of knowledge available internationally.In this time of transformative change, we must reflect upon the relevance of the leading normative and cognitive systems of knowledge production and the decision frameworks they support. By doing so, we may reconcile for the asymmetries in social realities and cultural meaning, to build a more inclusive conceptualization of agricultural sustainability. Conceptual growth comes from negotiating meaning, sharing multiple perspectives of reality and creating a shared vision. In this study, we take a constructivist approach to explore the socio-cultural constructs through which cover crop knowledge has been developed, and how this social quality shapes our appreciation of this practice. We bring to light the disparate nature of the two leading scientific rationales for cover cropping, and their distinct visions of sustainable agriculture: anthropogenic conservation and biocentric preservation. The anthropogenic conservation approach is directed towards natural resource conservation for sustainable consumption. The ecological preservation approach assumes nature is inherently valuable, irrespective of immediate profitability. This leaves the risks and challenges of interpretation to practitioners, who must make multi-disciplinary decisions. Although cover crop ES are intricately systematized, the design of the practice is not. We suggest that the increased adoption of cover cropping is held by a valuation optimization bind whereby confounding ES value systems and considerable lags in seed design optimization delays cover crop utilization.
Unlocking this bind will require the engagement of the seed industry to develop optimized seed varieties and to ensure their widespread distribution. This can only occur with a two-way conversation of value systems between researchers and practitioners, over the rationale of cover cropping as a pillar of sustainable agriculture.After more than a century of cover crop field research, scientific discourse has acknowledged the important contribution of cover cropping to the sustainability of food systems. The rationale behind the use of multi-species cover crops in support of agricultural sustainabilityis based on Tilman’s diversity-productivity theory. Tilman demonstrated that increased diversity could augment cover crop primary productivity and ES through higher resource use efficiency . Thereby, ecosystem processes are not only dependent on the identity of species, but also on the number of species within a given ecosystem. Although initial studies suggested increased productivity with up to five species within an ecosystem, later work demonstrated benefits with up to 16 species . These studies were originally applied to natural ecosystems and then to cover crop studies for agroecosystems . More recent research demonstrates that, beyond improvements in resource use efficiency, increased diversity may benefit ecosystem functioning by supporting diverse plant functional traits . These recent findings highlight opportunities to align cover crop seed selection and design to meet differential conceptualizations of agricultural sustainability. The outcomes of cover cropping have been broadly introduced across the scientific literature as a cumulative suite of ecosystem services: soil retention, pollinator habitat provision, weed control, improved soil physical properties, carbon sequestration, biocontrol services, enhanced water quality and improved nutrient cycling . Recent literature demonstrates that cover crop services occur in bundles . However, comprehensive studies verifying the co-occurrence of these many services remain scarce . Managing for the co-occurrence of multiple ecosystem services holds challenges—for instance, mowing N-rich vegetative covers to improve nutrient cycling may be incompatible with the provision of floral resources to increase pollinators. In turn, promoting flowering of cover cropsmay come at the cost of higher water consumption for an orchard. Perennial agro-ecosystems provide unique opportunities to explore the benefits of a wide variety of cover crop uses and functions . Perennial systems represent an enormous diversity of cropping systems, varying in planting design , harvest strategies and pruning . These diverse agronomic practices reflect the different climates, soil types and economic contexts of perennial production systems and have immediate implications for the management of cover crops and their associated ES . These differences in management directly influence cover crop management, including the timing of cover crop seeding, the feasibility of berm cover, the degree of soil surface coverage and the ease of mowing operations . Compared to annuals or biennials, the perennial nature of woody and vine systems provides opportunities to study cover crops across multiple seasons and to explore different termination dates. In perennial systems, cover cropping can potentially fulfil a diversity of functions within these systems , and take different forms, dutch buckets based on varying ecosystem service valuation systems. Although ecological rationales for cover cropping have been elucidated, the implementation of the practice lags. There has been slow and limited adoption of cover cropping in many parts of the world . This disconnect is important because to address societal imperatives , the widespread adoption of sustainable agricultural approaches must occur, and cover cropping is a cornerstone practice. We believe a major gap between the establishment of scientific evidence and the actual uptake of sustainable agronomic practices is hindering progress. We suggest that lags in cover crop adoption reveal a mismatch between the scientific discourse and the relevance of the practice to growers. Surveys and focus group studies of practitioners have explored key factors involved in the decision to use cover crops.
These factors include barriers and motivators . Although the literature contains reports on the logic of practitioners for cover cropping, very little work has been done on the production of scientific knowledge, in which information can be similarly systematized to reflect scientists’ values. We suggest that the dissemination of cover crop knowledge from scientists to extensionists and stakeholders may reflect differential value systems, which obscure the benefits of multi-species covers and penalize them for economic constraints. We consider that lags in cover crop adoption are not solely due to knowledge gaps and uncertainties, but are the result of differing ES valuation systems and, particularly, different prioritizations of economic profitability, relative to other ecosystem services. A large body of literature has attempted to create a consensus in terms of a common, coherent definition of sustainability . However, some claim that sustainability as a concept is inherently malleable, due to its socio-cultural foundation and the existence of differing environmental realities . Hence, the meaning of sustainability exists on a spectrum of interpretations.Ecosystems services refer to the many additional services beyond food production, which society gains from agroecosystems. We propose that the ways in which ecosystem services are valued in cover crop assessments reflect different conceptualizations of agricultural sustainability. In the first section, we provide a literature review of cover crop developmental history, to consider how the development of the practice has historically reflected shifts in societal preferences and sustainability goals. In the second section, we conduct a meta-analysis of cover crop literature conducted in perennial systems and ask whether the nature of the ecosystem services measured within cover crop studies are dependent on commodity type. We ask how the malleability of cover crop assessment structures is reflected in the selection of cover crop plant species presented in the scientific literature. We consider that acknowledging the differential interpretations of sustainability expressed in the diverse uses of cover crops is key to the future development of the practice.To contextualize perennial field research within the broader history of cover crop research, we performed a detailed literature review. We studied the socio-cultural contexts in which different uses of cover cropping were developed, as well as shifts in cover crop designs in response to changes in societal goals. In this review section, we consider cover cropping as applied more broadly to both annual and perennial agroecosystems. We considered that cover crop developments in annual systems largely contributed to those of perennial systems. We explored the United States’ history specifically as a case study of cover crop research and development. Our historical review begins in 1900, when the use of “cover crop” as a term was first recorded. However, we recognize that this practice is ancient, with records of cover cropping dating backover a millennium. Our analysis considered existing cover crop reviews, particularly the works of Bugg and Waddington , Groff , Hartwig and Ammon , Peshin et al. , and Altieri and Schmidt , as well as more eco-sociological works, such as the work of Cochrane . In studying these works and others, we focused on socio-economic events and scientific discoveries, which influenced the emergence of specialized cover crop uses, particularly nutrient management and biological management applications. In doing so, we considered the development of cover crop rationales across specialized scientific disciplines, and how their associated methodological approaches may have shaped the design and uses of cover cropping.Our analysis included only studies conducted under field conditions. Cover cropping was defined as a vegetative cover within orchard alleyways and also included research where tree berms were seeded. Studies where cover crops were not integrated within the orchard, such as hedge row trials, were excluded. We defined “perennial agro-ecosystems” as land-use systems, in which woody and vine perennials are managed as agricultural crops. Our definition overlaps with certain definitions of “agroforestry systems” , but does not include linear agroforestry systems . Our analysis did not include creeping vine crops or herbaceous climbing plants, such as vanilla , hops and cucumber . Following inclusion, 285 cover crop articles remained, of which the source references are detailed in Appendix B – Supplementary Information 2.4. Although most material was peer-reviewed, our selection also included land-grant university extension articles and conference materials.